Highly corrosion-resistant zirconium alloy for use as nuclear reactor fuel cladding material

ABSTRACT

A corrosion resistant zirconium alloy for use as a nuclear reactor cladding material. This alloy consists essentially of, on a weight % basis 0.02-1.7 % Sn, 0.19-0.6% Fe, 0.07-0.4% Cr, 0.01-0.2% Ta, and optionally 0.05 up to less than 0.5% Nb, the balance being Zr and incidental impurities, provided that the content of the nitrogen in the form of incidental impurities is no more than 60 ppm.

This application is a Continuation, of application Ser. No. 07/300,230,filed Jan. 23, 1989 (abandoned); which is a Continuation-In-Part of Ser.No. 07/072,411 filed Jul. 13, 1987 (abandoned).

FIELD OF THE INVENTION

The present invention relates to a zirconium alloy suitable for use asthe material of a nuclear reactor fuel cladding that is to be exposed tohot pressurized water or steam.

TECHNICAL BACKGROUND

Pressurized-water reactors (PWR) in nuclear power plants employ fuelcladdings made of zirconium alloys, of which Zircaloy 4 having thefollowing composition is typical:

1.2-1.7% Sn;

0.18-0.24% Fe;

0.07-0.13% Cr; and

the balance being zirconium and incidental impurities (the percentagebeing based on weight as hereinafter).

With a view to improving the economy of the operation of nuclear powerplants, efforts are being made to maximize the efficiency of fuel burnupand this has led to the need for fuel claddings to stay in the reactorfor a longer period. But this need cannot be met by the conventionalfuel claddings made of zirconium alloys typically Zircaloy 4, becausethey do not have sufficient corrosion resistance to withstand prolongedexposure to the atmosphere in the reactor.

Under these circumstances, the present inventors conducted studies inorder to develop a zirconium alloy that exhibits superior corrosionresistance when used as a nuclear reactor fuel cladding material, thestudies being particularly addressed to modification of Zircaloy 4. As aresult, the present inventors found that an alloy additionallycontaining Ta, and optionally Nb, too, in the composition specifiedabove had such improved corrosion resistance that it was suitable foruse as a nuclear reactor fuel cladding material over a prolonged period.

SUMMARY OF THE INVENTION

The present invention has been accomplished on the basis of this findingand provides a zirconium alloy for use as a nuclear reactor fuelcladding material that contains:

0.2-1.7% Sn, preferably 0.2-1.15% Sn,

0.19-0.6% Fe, preferably 0.19-0.24% Fe,

0.07-0.4% Cr, preferably 0.07-0.13% Cr,

0.01-0.2% Ta, and optionally

0.05 up to less than 0.5% Nb,

the balance being zirconium and incidental impurities, provided that thecontent of the nitrogen in the form of incidental impurities is no morethan 60 ppm.

DETAILED DESCRIPTION OF THE INVENTION

The tin (Sn), iron (Fe) and chromium (Cr) components present incombination in the zirconium alloy of the present invention serve toprovide improved corrosion resistance. If the amount of any one of thesethree components is less than the lower limit specified above, thedesired level of corrosion resistance is not ensured. On the other hand,if the amount of Sn exceeds 1.15%, the corrosion resistance of theresulting alloy tends to decrease. This tendency is encouraged if the Snamount exceeds 1.7%. The Sn content in the alloy of the presentinvention is therefore limited to the range of 0.2-1.7%, preferably0.2-1.15%. If the amounts of Fe and Cr exceed 0.24% and 0.13%,respectively, the corrosion resistance of the alloy also tends toslightly decrease. This tendency is encouraged if the amounts of Fe andCr exceed 0.6% and 0.4%, respectively. The Fe content is thereforelimited to the ranges of 0.19-0.6%, preferably 0.19-0.24%, and the Crcontent is limited to the range of 0.7-0.4%, preferably 0.07-0.13%.

Tantalum also serves to provide improved corrosion resistance. If itscontent is less than 0.01%, the intended effect for improving corrosionresistance is not attained. If, on the other hand, the content of Taexceeds 0.2%, absorption of neutrons is so much increased as to inhibitnuclear reactions. Therefore, the content of Ta in the alloy of thepresent invention is limited to be within the range of 0.01-0.2%.

The alloy of the present invention may optionally contain niobium which,if present in combination with tantalum, contributes to furtherimprovement in corrosion resistance. If the content of Nb is less than0.05%, the desired effect for improving corrosion resistance will not beattained. If, on the other hand, the content of Nb is 0.5% or greater,strong neutron absorption also occurs as in the case of the presence ofexcessive tantalum. In addition, the presence of excessive niobium leadsto the formation of an increased amount of precipitate in the alloy thatreduces its workability. Therefore, the content of niobium in the alloyof the present invention is limited to be within the range of 0.05 up toless than 0.5%.

If the content of nitrogen in the form of incidental impurities exceeds60 ppm., high corrosion resistance cannot be ensured even if thecontents of Sn, Fe, Cr, Ta and Nb are within the ranges specified above.The nitrogen content in the form of incidental impurities is thereforelimited to be no more than 60 ppm.

Example

The zirconium alloy of the present invention is hereunder described morespecifically by means of an example.

A zirconium sponge having a purity of no less than 99.8%, and Sn, Fe,Cr, Ta and Nb powders each having a purity of 99.9% were provided asstarting materials. These starting materials were blended inpredetermined proportions and the resulting mixes were pressed intocompacts. The compacts were melted in an arc furnace and formed intobuttons, which were then hot-forged at a temperature of 600° C. for adraft of 50% and heated at 1,080° C. Thereafter, the buttons werewater-quenched and pickled in a salt bath, followed by cold rolling fora draft of 50%, holding at 630° C. for 2 hours to effectrecrystallization and annealing, another cold rolling for a draft of50%, and holding at 450° C. for 2 hours to effect stress-relievingannealing. Finally, the buttons were pickled and polished. As a result,test specimens for Zr alloy Nos. 1 to 12 of the present invention andcomparative Zr alloy Nos. 1 to 7 were prepared; these specimens had thecompositions shown in Table 1 and measured 20 mm wide, 40 mm long and0.5 mm thick.

Comparative Zr alloy Nos. 1 to 7 were such that one of their components(marked with an asterisk in Table 1) was outside the compositional rangespecified by the present invention.

All test specimens were subjected to an out-of-pile corrosion test in acommon static autoclave in steam at 400° C. and at 105 kg/cm². After 200days, the corrosion weight gain that had occurred in each test specimenwas measured. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________            Composition (wt %)           Corrosion                                                           N   Zr +  weight gain                              Sample  Sn  Fe  Cr   Ta Nb (ppm)                                                                             impurities                                                                          (mg/dm.sup.2)                            __________________________________________________________________________    Zr alloy                                                                      of this invention                                                              1      0.21                                                                              0.20                                                                              0.10 0.048 37  bal.   99                                       2      0.52                                                                              0.21                                                                              0.092                                                                              0.012 32  "     128                                       3      0.51                                                                              0.20                                                                              0.097                                                                              0.049 40  "     104                                       4      0.50                                                                              0.20                                                                              0.11 0.071 32  "     101                                       5      0.51                                                                              0.21                                                                              0.10 0.18  33  "     113                                       6      0.70                                                                              0.20                                                                              0.092                                                                              0.58  30  "     108                                       7      0.75                                                                              0.193                                                                             0.013                                                                              0.082 39  "     105                                       8      1.19                                                                              0.21                                                                              0.10 0.013 38  "     136                                       9      1.13                                                                              0.21                                                                              0.098                                                                              0.053 34  "     121                                      10      1.41                                                                              0.20                                                                              0.12 0.052 44  "     136                                      11      1.63                                                                              0.20                                                                              0.11 0.095 51  "     134                                      12      1.66                                                                              0.20                                                                              0.095                                                                              0.082 41  "     131                                      13      1.02                                                                              0.191                                                                             0.10 0.047 32  "     114                                      14      1.06                                                                              0.36                                                                              0.11 0.047 33  "     125                                      15      0.99                                                                              0.56                                                                              0.11 0.050 37  "     139                                      16      1.03                                                                              0.21                                                                              0.075                                                                              0.049 33  "     112                                      17      1.04                                                                              0.20                                                                              0.22 0.052 35  "     114                                      18      1.00                                                                              0.22                                                                              0.36 0.055 36  "     148                                      19      1.02                                                                              0.33                                                                              0.21 0.051 52  "     124                                      20      1.03                                                                              0.41                                                                              0.32 0.048 46  "     136                                      21      1.05                                                                              0.55                                                                              0.37 0.048 33  "     164                                      22      1.01                                                                              0.21                                                                              0.10 0.053                                                                            0.054                                                                            31  "     122                                      23      1.00                                                                              0.22                                                                              0.12 0.051                                                                            0.11                                                                             40  "     115                                      24      1.02                                                                              0.21                                                                              0.10 0.046                                                                            0.23                                                                             36  "     113                                      25      0.97                                                                              0.20                                                                              0.10 0.054                                                                            0.48                                                                             35  "     127                                      26      0.24                                                                              0.21                                                                              0.10 0.031                                                                            0.052                                                                            32  "     102                                      27      0.81                                                                              0.20                                                                              0.092                                                                              0.029                                                                            0.32                                                                             38  "     110                                      28      0.48                                                                              0.192                                                                             0.10 0.091                                                                            0.16                                                                             30  "     107                                      29      0.53                                                                              0.38                                                                              0.29 0.095                                                                            0.14                                                                             34  "     119                                      30      0.51                                                                              0.57                                                                              0.35 0.084                                                                            0.17                                                                             39  "     148                                      Comparative                                                                   Zr alloy                                                                       1      0.14*                                                                             0.20                                                                              0.10 0.040 35  "     160                                        2     1.91*                                                                             0.191                                                                             0.10 0.050 32  "     172                                       3      1.21                                                                              0.15*                                                                             0.098                                                                              0.053 39  "     145                                       4      1.01                                                                              0.76*                                                                             0.11 0.045 37  "     163                                       5      1.05                                                                              0.193                                                                             0.051*                                                                             0.081 40  "     149                                       6      1.06                                                                              0.21                                                                              0.55*                                                                              0.052 31  "     198                                       7      1.46                                                                              0.21                                                                              0.10 --*                                                                              --*                                                                              33  "     199                                       8      0.98                                                                              0.22                                                                              0.10 --*                                                                              --*                                                                              38  "     151                                       9      1.02                                                                              0.21                                                                              0.12 0.055                                                                            0.63*                                                                            40  "     179                                      10      1.01                                                                              0.194                                                                             0.11 0.060                                                                            0.11                                                                              73*                                                                              "     234                                      __________________________________________________________________________     *outside the scope of this invention                                     

As the data in Table 1 show, Zr alloy Nos. 1 to 30 of the presentinvention exhibited superior corrosion resistance over comparative Zralloy Nos. 1 to 10 in which one of the components was outside thecompositional range specified by the present invention.

The zirconium alloy of the present invention exhibits improved corrosionresistance under the conditions to which nuclear reactor fuel claddingsare exposed and, therefore it will offer many industrial advantages suchas the one that fuel claddings made of this alloy can be put tocommercial service over a very long period.

What is claimed is:
 1. A corrosion resistant zirconium alloy for uses asa nuclear reactor fuel cladding material consisting essentially of on aweight % basis0.2-1.7% Sn, 0.19-0.6% Fe, 0.07-0.4% Cr, 0.01-0.2% Ta, thebalance being Zr and incidental impurities, provided that the content ofnitrogen in the form of incidental impurities is no more than 60 ppm. 2.The zirconium alloy of claim 1, further comprising from 0.05 to lessthan 0.5% Nb.
 3. The zirconium alloy of claim 2, wherein the Fe contentis 0.19-0.24% and the Cr content is 0.07-0.13%.
 4. The zirconium alloyof claim 2, wherein the Sn content if 0.2-1.15%.
 5. The zirconium alloyof claim 3, wherein the Sn content is 0.2-1.15%.
 6. The zirconium alloyof claim 1, wherein the Sn content is 0.2-1.15%.
 7. The zirconium alloyof claim 1, wherein the Fe content is 0.19-0.24% and the Cr content is0.07-0.13%.